In order to reduce industrial pollution, it is necessary to have available processes and products for removing toxic waste products, such as the oxides of nitrogen which are produced in processes of combustion of all kinds.
One of the classical methods for the removal of this kind of impurity consists in reducing the oxides of nitrogen. The reaction takes place in the presence of catalysts the active areas of which may be formed from platinum metals and the metal oxides conventionally used for catalytic oxidation.
The reducing gas may be taken from any convenient source. In the case of plants which produce nitric acid by the catalytic oxidation of ammonia, since the starting material is ammonia, this can serve as a readily available source of reducing gas.
In heterogeneous catalysis it is well known that it is not enough merely to use active materials which initiate the various kinds of chemical reactions such as oxidation, hydrogenation and cracking but that in fact it is necessary to carry out the reaction in the most economically possible manner.
Industry is, therefore, seeking a catalyst which exhibits the greatest possible activity, that is to say a catalyst which can be used in small volume, a reactor of small dimensions and very much improved conditions of operation during the reduction of oxides of nitrogen, namely: a low temperature, a high throughput of effluent gases through the reactor, a high conversion and improved selectivity. It is also desirable to have a catalyst the useful life of which is compatible with the economics of the process.
Success in meeting these requirements depends upon the way in which the active areas of a solid catalyst participate in the reaction. More especially, the structure of the solid catalyst, its structure and the promoters present are, for a given catalyst, factors of great importance.
Catalysts having active areas which consist of platinum metals which ar prepared by processes involving impregnating or exchanging the platinum have a heterogenous structure constituted by the support upon which the platinum is distributed, and this results in an irregular distribution of the active areas and consequently in a limited effectiveness. Moreover, these catalysts are obtained from rare and expensive metals.